Hey there! Thanks so much for writing for me! I love the whole premise of this exchange, and I'm excited for whatever you write. Anything goes, and if you'd like to ignore these prompts and do your own thing, that's completely fine with me!
You can browse my past Dear Author letters for some idea of what I like and dislike in fics. For this particular exchange, most of my major do-not-wants fall into interrelated categories; I would greatly prefer no issue-fic, nor ambiguity for its own sake. In particular, I would rather not receive a gift whose primary focus is on how you or your characters know better or are more politically correct than the original author on any levels, nor one celebrating uncertainty or incomprehensibility of narratives as ends in themselves. This is not an endorsement of any of the widely-disparate authors in question, by any means, just things I don't prefer reading in exchanges. That being said, I also have nothing against unabashedly-meta stories, self-aware and clever stories within stories, etc, so please don't hold back if something ridiculously self-referential strikes, but don't feel compelled to go down that road either.
Okay, onto the fandoms! In several cases, I had a hard time choosing among which of several related fandoms to request; I've mentioned below which crossovers would be perfectly fine with me.
Celestial Emporium of Benevolent Knowledge
We all know about the animals, of course, so maybe a piece centered around the animals included in this classification? Or the animals not included in this classification if you're a perverse set theorist. But feel free to branch out and write about people trying to apply other categories in the Emporium to real life; how do they deal with plants? With stones? Household objects? Further abstractions? Crossovers with the First Encyclopaedia of Tlön or The Garden of Forking Paths would also be great!
Flying With the Cannons
I love sports stories, so anything involving the wonderful world of Quidditch would be awesome. If you'd like to ship Joey Jenkins with a teammate or rival or any one else, het or slash would both be fine, but I love gen too. Maybe something about luck finally turning the Cannons' way at last, although I'd prefer no cheaters getting away with cheating. Can be more fictional (Marvin Miggs crossover, anybody?) than RPF if that's your style.
The Band with Rocks In
I'm not very well-versed in the conventions of band fic, so adapting that genre to the Disc or doing something different are both fine. I'd prefer no inter-band ships, but Buddy/Susan in the original or altered timeline would both be great; band-focused gen is, of course, wonderful too. Whether refusing to yield their hard-earned profits or reading books about disc-communism, anything the Band gets up to will thrill me. And, of course, rock puns. I can't promise to recognize all the rock puns, but I would appreciate whatever makes me groan.
The Flame The Blade And The Heart
Any or all of the ships here is fine with me! Birgitte/Gaidal? Anselan/Barashelle? Rogosh/Dunsinin? (See here and linked pages for more.) If you'd rather do a gen fic instead, I'm fine with that (early days of Barashelle and Anselan's relationship, with them only being tentative coworkers?) I'd rather not see Birgitte or Gaidal shipped with other people. Crossovers with Killers of the Black Veil, or The Travels of Jain Farstrider (particularly in light of the Horn sounding at the end of "A Memory of Light") would be amazing.
The Hive-Queen and the Hegemon - Speaker for the Dead
This seems like a jumping-off point to explore many different alternate histories, or potential turning points during or after the Formic Wars. I generally don't like the motif of "oh, we thought this was just a serendipitous decision, but actually so-and-so had it planned all along," so maybe just random musings on how a small change could have ripple effects? (This seems like a good fandom for 5(+1) Things fic, for some reason, but that's just a suggestion.) I'd prefer not to get too heavy into themes of collective guilt, or at least not without some hopeful themes as well (I thought "Shadows in Flight" and even "Pastwatch" were surprisingly good at leaning to the "hopeful" rather than "your people are bad and you should feel bad" tropes, but I recognize that not everyone has the same tastes!)
The Horn of Joy — Matthew Maddox
Crossovers with Once More United=absolutely fine with me! I'd be up for future-fic or past-fic, although with this fandom, it's kind of hard to tell the difference. If you don't particularly care about these characters, some worldbuilding stuff about travels-by-unicorn would be cool. I'm not a fan of ship wars, unless it's as, you know, an "antiwar text." ;)